1. Field of the Invention
The present invention relates to wireless communication system technology. More particularly, the present invention relates to a method and apparatus for configuring a search space of a physical downlink control channel.
2. Description of the Related Art
In a wireless communication system, downlink transmission refers to a base station's transmission of a signal to a User Equipment (UE). A downlink transmission signal includes a data signal, a control signal and a reference signal (i.e., Pilot). Herein, the base station transmits downlink data in a Physical Downlink Shared Channel (PDSCH), or transmits downlink control information in a downlink control channel. Uplink transmission refers to transmission of a signal by the UE to the base station. An uplink signal also includes a data signal, a control signal and a reference signal. Herein, the UE transmits uplink data in a Physical Uplink Shared Channel (PUSCH), or transmits uplink control information in a Physical Uplink Control Channel (PUCCH). The base station may dynamically schedule PDSCH transmission and PUSCH transmission of the UE by a Physical Downlink Control Channel (PDCCH). In a 3GPP LTE system, downlink transmission is performed using Orthogonal Frequency Division Multiple Access (OFDMA) technology, and uplink transmission is performed using Signal-Carrier Frequency Division Multiple Access (SCFDMA) technology. In the 3GPP LTE system, the length of each wireless frame, which is equally divided into 10 subframes, is 10 ms and a downlink Transmission Time Interval (TTI) is defined as one subframe
FIG. 1 is a schematic diagram illustrating a subframe structure in a Long Term Evolution (LTE) system according to the related art.
Referring to FIG. 1, each downlink subframe includes two time slots. For a normal Cyclic Prefix (CP) length, each time slot contains 7 OFDM symbols; for an extended CP length, each time slot contains 6 OFDM symbols. In each subframe, the first n OFDM symbols, where n is equal to 1, 2 or 3, are used for transmitting the downlink control information including the PDCCH and other control information; the remaining OFDM symbols are used for transmitting the PDSCH.
Resource allocation is based on a Physical Resource Block (PRB), wherein one PRB contains 12 consecutive sub-carriers in the frequency domain and corresponds to one time slot in the time domain. Two PRBs in two time slots of the same sub-carrier in one subframe constitute a PRB pair. In each PRB pair, a Resource Element (RE) is a minimal unit of time and frequency resource. That is, the RE contains one sub-carrier in the frequency domain and contains one OFDM symbol in the time domain. REs may be respectively used for different functionalities. For example, some of the REs may be used respectively for transmitting a Cell specific Reference Signal (CRS), a user specific Demodulation Reference Signal (DMRS), a Channel State Information Reference Signal (CSI-RS) etc.
In the LTE system, multiple data transmission modes are defined. For example, in the downlink direction, the data transmission modes include a closed loop Multiple Input Multiple Output (MIMO) transmission mode, an open loop MIMO transmission mode, a transmitting diversity transmission mode etc. In the uplink direction, the transmission modes include a single antenna transmitting mode, a MIMO mode, etc. For one transmission mode, the system configures a normal Downlink Control Information (DCI) format used for normal data transmission in the transmission mode. At the same time, the base station configures the UE to check a fallback DCI format having a small number of bits and that is used for scheduling data in a conservative manner, e.g., transmitting diversity or data transmission through a single antenna, thereby having high reliability. In addition, DCI formats of the uplink transmission and the downlink transmission are different. That is, in one TTI, the UE needs to detect multiple possible DCI formats.
In the LTE system, DCI transmitted to different UEs or DCI of different functionalities is coded independently and transmitted. When the PDCCH is mapped to physical resources, a Control Channel Element (CCE) is taken as a unit, i.e., one PDCCH modulation symbol may be mapped to L CCEs where L is equal to 1, 2, 4, or 8, and L is also called a PDCCH aggregate level. Each CCE contains 36 REs. Since the PDCCH uses Quad-Phase Shift Keying (QPSK) as a modulation method, the base station may select the aggregate level of the CCE for transmitting the PDCCH according to the number of bits of the control information and the UE's link condition.
Herein, if each UE is configured with a unique PDCCH, when the quantity of UEs exceeds the quantity of PDCCHs, a problem is caused that the PDCCHs of the UEs block each other. Otherwise, if all of the PDCCHs may be allowed to be configured to all of the UEs, the UEs need to be configured to detect all of the possible PDCCHs. As a consequence, the UE's complexity is increased and a false alarm rate is caused to be increased. Thus, in the LTE system, the UE is configured to detect the PDCCH at multiple possible locations, which are called a UE search space. The base station transmits the PDCCH on one of the multiple possible locations configured for UE detection. By blindly detecting the multiple locations configured by the base station, the UE may obtain the control information transmitted by the base station on one of the locations. In the LTE system, the UE needs to detect the PDCCH respectively in a cell Common Search Space (CSS) and a UE specific Search Space (USS). Herein, the PDCCH in the CCS is usually used for scheduling system broadcast information etc., and the PDCCH in the USS is usually used for dynamically scheduling the PDSCH and PUSCH of the UE.
According to the above description, the UE needs to detect the CSS and the USS respectively, and in each search space, the UE needs to detect multiple possible DCI formats. In the LTE system, the number of PDCCH candidates corresponding to each aggregate level for each type of the DCI formats is defined, as shown in Table 1. Herein, for the same search space, the number of PDCCH candidates needing to be detected for various DCI formats are the same.
TABLE 1Search Space S(L)The number of the PDCCHTypeAggregate Level Lcandidates M(L)UE specific16264282Cell common4482
According to Table 1, when the aggregate level is 1, the number of the aggregate PDCCHs is 6. Thus, the UE needs to blindly detect the PDCCH on 6 possible locations.
In an enhanced version of LTE, the PDCCH may become a system performance bottleneck since it must support multi-cell joint transmission or a heterogeneous network. In order to support greater capacity of the control channel, and to support interference collaboration of control channels of multiple cells, an enhanced PDCCH (E-PDCCH) is proposed.
FIG. 2 is a schematic diagram illustrating an existing multiplexing method of an E-PDCCH according to the related art.
Referring to FIG. 2, the E-PDCCH is mapped to a data region of the subframe and multiplexed with the PDSCH by using Frequency Division Multiplexing (FDM).
As shown in FIG. 2, it is assumed that the E-PDCCH is started from the OFDM symbol that is adjacent to the PDCCH (actually the E-PDCCH may alternatively be fixedly started from one OFDM symbol configured by a high layer), and the E-PDCCH occupies a certain number of OFDM symbols. The base station may notify the UE of the PRB pair used for transmitting the E-PDCCH by a higher layer signal which may be cell specific or may be specially transmitted to each UE respectively, and for different UEs, the PRB pairs used for the E-PDCCH may be different.
According to a method for mapping E-PDCCH resources, the E-PDCCH may be divided into a localized E-PDCCH and a distributed E-PDCCH. When the accurate Channel Quality Indication (CQI) of different frequency sub-bands of the UE can be obtained, the base station may choose to transmit the E-PDCCH in a suitable PRB pair to obtain frequency scheduling gain, i.e., the localized E-PDCCH. Accordingly, when the accurate CQI of the UE is not obtained, the base station has to transmit the E-PDCCH in multiple PRB pairs to obtain frequency distribution gain, i.e., the distributed E-PDCCH. The distributed E-PDCCH may also be used in a condition that the E-PDCCH needs to be transmitted to multiple UEs.
The E-PDCCH is also composed of Enhanced-CCEs (E-CCEs), which correspond to the CCEs that constitute the PDCCH. The UE also needs to detect one or multiple E-PDCCH candidates in a certain search space corresponding to the search space of the PDCCH.
In each PRB pair, the number of REs that may be practically used for transmitting the E-PDCCH is variable and relies on multiple situations such as:                a. the number of OFDM symbols in a backward compatible control area, i.e., the number of the OFDM symbols occupied by the downlink control information;        b. the number of REs occupied by the CRS that is transmitted in the PDSCH area for the normal subframe but is not transmitted in the PDSCH area for a Multicast Broadcast Single Frequency Network (MBSFN) subframe;        c. the number of REs used for the DMRS; and        d. whether the CSI-RS is transmitted, etc.        
The variation of the available REs in the PRB pair causes variation in the size of the E-CCE. The variation range of the number of REs of the E-CCE may be reduced by adjusting the number of the divided E-CCEs according to the number of available REs, but this cannot thoroughly avoid the variation of the number of REs.
FIG. 3 is a schematic diagram illustrating methods for dividing the localized E-PDCCH into E-CCEs according to the related art.
Referring to FIG. 3, it is assumed that each PRB pair is divided into 4 E-CCEs. In example 1, a backward compatible control channel occupies 3 OFDM symbols, a normal subframe structure is used, 4 CRS ports are configured, and the CSI-RS is configured. Thus, only 84 REs may be used for the E-PDCCH and each E-CCE only has 21 REs, on average. In example 2, the backward compatible control channel occupies 2 OFDM symbols, an MBSFN subframe structure is used, 4 CRS ports are configured, and no CSI-RS is configured. Thus, there may be 120 REs used for the E-PDCCH, and each E-CCE contains 30 REs, on average. It can be seen that the sizes of the E-CCE in the two examples are substantially different. Actually, according to the simple method for dividing the E-CCE by subcarriers, even in the same example, the sizes of the E-CCEs are not completely equal.
Accordingly, there is a need for a method and apparatus for configuring a search space of a downlink control channel to increase the flexibility of base station scheduling and reduce the possibility that the E-PDCCHs of different UEs block each other.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.